November 21, 2008
image by Wes Higgins, Tecumseh, Oklahoma
A north up view of Alphonsus provides a chance to consider rims of large, degraded craters. We know from Tycho and Copernicus that young large craters have a series of terraces that stair-step down to the floor. In Alphonsus the terraces are largely gone, but a remnant of a steep scarp is indicated in the NW by the curved line of shadow below the rim crest. This line of probable old rim crest also can be recognized by the evenness of bright hills on the NE side of the rim. In most places, elongated secondary crater chains of Imbrium ejecta, and the profound shaking that must have accompanied the basin's formation, have reduced the rim to a formless mass. But the rim still clearly defines the crater. On the west, you can see that the 20 km wide rim (half the diameter of Alpetragius at lower left) is well defined, and it's shadows on the east side reveal that it rises significantly above the surrounding terrain.
08-23-08 10:21 UT. 18" Reflector, Infinity 2-1m camera, MAP(3,781), stack of 87 frames
Rükl plate 44
I cropped the image above from a larger view.
Yesterday's LPOD: Half the Henrys
Tomorrow's LPOD: A Dome On the Limb